docs($interval): improve fn description

If no additional arguments are passed, the function is called with the current iteration count.

Closes #15503

src/ng/interval.js

@@ -33,7 +33,8 @@ function $IntervalProvider() {
       * appropriate moment.  See the example below for more details on how and when to do this.
       * </div>
       *
-      * @param {function()} fn A function that should be called repeatedly.
+      * @param {function()} fn A function that should be called repeatedly. If no additional arguments
+      *   are passed (see below), the function is called with the current iteration count.
       * @param {number} delay Number of milliseconds between each function call.
       * @param {number=} [count=0] Number of times to repeat. If not set, or 0, will repeat
       *   indefinitely.